The Dynamic Host Configuration Protocol (DHCP) provides an alternative to static IP addressing by enabling automatic configuration of IP-related parameters when a DHCP client becomes active on a network. In particular, DHCP enables a DHCP server to assign an IP address to the DHCP client, e.g., from a pool of unused and available IP addresses. A description of DHCP is provided in a publication entitled “RFC 2131—Dynamic Host Configuration Protocol” by R. Droms, Bucknell University, dated March 1997, the teachings of which are hereby incorporated by reference in their entirety.
In some situations, DHCP relay agents convey DHCP messages between DHCP clients and DHCP servers. For example, suppose that a DHCP client is on Subnet A and a DHCP server is on Subnet B which is separated from Subnet A by a router. In this situation, the router can be configured to run a DHCP relay agent which forwards the DHCP messages between the subnets, e.g., the DHCP relay agent can forward a DHCPDISCOVER message broadcasted on Subnet A by the DHCP client to the DHCP server on Subnet B, and so on.
A “Relay Agent Information” option (i.e., option code 82) is available which enables a DHCP relay agent to insert a DHCP option with relay agent information onto a DHCP message sent from a DHCP client to a DHCP server. The DHCP server can then use this relay agent information during the process of assigning an IP address to the DHCP client. In accordance with the protocol, the DHCP server echoes the DHCP option back to the DHCP relay agent when providing a response to the DHCP message thus enabling the DHCP relay agent to use the same relay agent information when conveying the response back to the DHCP client. For example, a DHCP relay agent terminating a switched or permanent circuit can use the echoed relay agent information to forward the response back to the proper circuit.